Magnetron sputtering of rotating targets is well known and is used extensively for producing a wide variety of thin films on a wide variety of substrates. In the most basic form of rotating-target magnetron sputtering, the material to be sputtered is either formed in the shape of a tube or is adhered to the outer surface of a support tube made of a rigid material. A magnetron assembly is disposed within the tube and supplies magnetic flux, which permeates the target such that there is adequate magnetic flux at the outer surface of the target. The magnetic field produced by the magnetron assembly is designed in a way such that it retains electrons emitted from the target so as to increase the probability that they will have ionizing collisions with the working gas, hence enhancing the efficiency of the sputtering process.
It is becoming increasingly important to compensate for target erosion effects because it is desirable to increase target thickness and operate sputter processes under more sensitive process conditions. The desire for thicker targets is largely driven by fabrication costs of ceramic targets, but is also desirable in order to have a greater inventory of usable material inside the sputter coater in order to run longer coating campaigns. The need to run processes in more sensitive process conditions is driven by the desire to get higher deposition rates, in reactive mode sputtering, and/or to finely control film chemistry.
Fabrication cost for targets of some materials, in particular ceramic transparent conductive oxide (TCO) materials, are relatively high in comparison to the cost of the raw materials. To improve the economy of these targets, it is desirable to increase the thickness of the target material. In this way, the target will have significantly more usable material while adding only minimally to the overall cost of the target, as the fabrication cost does not change significantly. The only significant cost increase is due to the additional raw material used. In addition, thicker targets have the added benefit of allowing longer production campaigns between target changes.
Increasing the target thickness too much, however, can result in inadequate magnetic flux at the target surface when using standard magnetron assemblies. Magnetron designs with higher magnetic flux have recently been introduced to provide the higher magnetic flux required for the thicker targets.
In the case of reactive magnetron sputtering, metallic targets are sputtered in an atmosphere that contains reactive gas such as oxygen or nitrogen. The sputtered material reacts with the reactive gas in order to form a film comprised of compounds of the target material and the reactive gas. The reactive gas also reacts with the target surface, thereby forming reacted compounds on the target surface. The surface compounds greatly reduce the ablation rate. In order to improve the sputtering efficiency, the amount of reactive gas may be carefully controlled so as to minimize the target surface reactions while still achieving the desired film chemistry. In some cases, the processes need to be controlled such that the chemistry of the film is sub-stoichiometric.
This fine control over the process gas makes the process sensitive to small perturbations. The industry has seen considerable technological advances in power delivery and process gas control that have minimized many of the process perturbations. Nevertheless, little has been done to minimize variations in the magnetic confinement of the plasma. As the target erodes, the working surface gets closer to the magnetic assembly and the magnetic field becomes stronger. This changes the confinement of the plasma, altering the dynamics of the sputtering process. This presents a challenge in maintaining long-term stability of the process.
The typical magnetron assembly for rotating cathodes comprises three substantially parallel rows of magnets attached to a yoke of magnetically conductive material, such as steel, that helps complete the magnetic circuit. The direction of magnetization of the magnets is radial with respect to the major axis of the sputtering target. The center row of magnets has the opposite polarity of the two outer rows of magnets.
Magnetic flux of the inner and outer rows of magnets is linked through the magnetically conductive yoke, on one side of the magnets. On the other side of the magnets, opposite the yoke, the magnetic flux is not contained in a magnetically conductive material. Hence, the magnetic flux permeates substantially unimpeded through the target, which is substantially non-magnetic. Thus, two arc-shaped magnetic fields are provided at and proximate to the working surface of the target. These fields retain the electrons and cause them to drift in a direction perpendicular to the magnetic field lines, which is parallel to the rows of magnets. This is known as the E×B drift. In an ordinary arrangement, this drift path is also parallel to the major axis of the target.
Additionally, the outer rows of magnets are slightly longer that the inner row of magnets, and additional magnets, of the same polarity as the outer rows, are placed at the ends of the assembly between the two outer rows creating the so-called “turn-around” areas of the drift path. This has the effect of connecting the two drift paths, hence forming one continuous ovular “racetrack” drift path. This optimizes the retention of the electrons and therefore optimizes the efficiency of the sputtering process.
As the target erodes, the working surface comes closer to the magnet assembly, and the intensity of the magnetic field, at the working surface, increases in a non-linear fashion. For finely controlled processes it very desirable to modify the magnetic field, as the target erodes, so as to minimize variability of the process, thereby making the process easier to control over the course of the target life.
The need for changing the magnetic field as the target erodes is well known, and has been accomplished in the case of planar sputtering cathodes. The need for an adjustable magnetron for rotating cathodes has gone unsatisfied, however, because the geometry and mechanical structure of the cathodes make the task especially challenging.